A relative movement of mechanical parts, such as sliding or rolling, inevitably incurs friction and wear. Depending on the contact parameters, two main lubrication regimes, separated by the mixed regime, can be defined. On the one hand, during the hydrodynamic lubrication regime, i.e. at higher speeds or lower loads, the counter surfaces are fully separated by a lubricating film.
On the other hand, during the boundary regime, i.e. at lower speeds or higher loads, the lubricating film loses its integrity and contacts between asperities occur. Macroscopically, the boundary regime is characterised by a significant increase in friction coefficient values and generally by higher wear rates.